To condition air to be supplied to a passenger compartment, prior art air conditioning systems include a refrigeration plant for cooling the air and a glycol-air heat exchanger for heating the air. These systems also typically include glycol-air heat pumps and air-air heat pumps.
Air conditioning units having glycol-air heat exchangers are disadvantageous in that when ambient air temperatures fall below 10° C., cooling water, as well as a glycol temperature cannot reach the temperature level required for comfortably heating the passenger compartment. Against a backdrop of increasing employment of vehicle drive systems which release a minimal amount of heat, such as electric or fuel cell drives, use of additional heaters is necessary.
It is inherent in glycol-air heat pumps that the cooling water of the internal combustion engine is utilized as heat source. However, heat extraction from the cooling water leads to the engine being operated at low temperatures for longer periods of time, which is undesirable.
If air-air heat pumps are used as heaters, ice builds up at a gas cooler or condensator at certain temperatures. Control of the system can prevent icing, but can also result in a minimization of heat output from the heat pump. If ice build-up at the gas cooler or condensator is permitted, the heat pump can be actively defrosted by operating the refrigerant circuit as an A/C-plant for a short time. However, this may result in a minimization of mean heat output of the heat pump.
Frequently, heat pump systems cannot dehumidify and heat the air to be fed into the passenger compartment at the same time. Therefore the systems cannot be operated with recirculated air from the passenger compartment, which may lead to undesired fogging of the windows. Additionally, heat pump systems delivering output to the engine cooling circuit frequently are insufficiently dynamic and efficient.
An air conditioning unit for vehicles is disclosed in DE 101 63 607 A1, hereby incorporated herein by reference in its entirety, having an integrated heat pump for cooling air and heating air, wherein refrigerant is circulated in a circuit and heat transmission is facilitated to air flowing into a passenger compartment. In this process, energy is fed directly into the air flowing into the passenger compartment by use of the integrated heat pump. This is facilitated by use of refrigerant-passed heat exchangers disposed in the vehicle's air conditioning box by cooling and optionally condensing the refrigerant for heating and evaporating the refrigerant for cooling. Next, the integrated heat pump energy is fed into the air flowing into the passenger compartment indirectly over heat carrier liquids. This is facilitated by use of liquid/air heat exchangers disposed in the vehicle's air conditioning box, by cooling the secondary refrigerant for heating the air and by heating the secondary refrigerant for cooling the air. It is a disadvantage of this invention that in heat pump mode of operation, the internal heat exchanger is not passed on the high-pressure side and a great number of components are required.
DE 101 26 257 A1, hereby incorporated herein by reference in its entirety, discloses a heating/cooling circuit for vehicles, whereby the heating/cooling circuit comprises an evaporator for cooling air to be fed to an interior, a heating heat exchanger for heating the air to be fed to the interior, an external heat exchanger with a compressor for discharging a refrigerant, a first expansion element allocated to the evaporator, a second expansion element allocated to the external heat exchanger, and refrigerant conduits connecting the components to each other. The invention provides means to adjust the pressure of the refrigerant in the external heat exchanger between highest and lowest system pressures to establish an effective, energy-saving, demand-oriented dehumidification of the air in cooling, heating and reheating modes of operation. Loading of the external heat exchanger with medium pressure, however, results in minimized performance of the external heat exchanger and a minimized efficiency of the heating/cooling circuit.
It would be desirable to produce an air conditioning unit for a combined refrigeration plant and heat pump mode of operation for vehicles, wherein a power consumption required to heat/cool the passenger compartment is minimized and a heating/cooling facilitated thereby is maximized.